Boil the sweet potatoes in a large pot of boiling water until tender, about 25 minutes. Allow to cool down, then peel.
Place the sweet potatoes in a large mixing bowl along with the butter, cream, agave, cinnamon, nutmeg, almond extract, vanilla extract, salt and lemon zest. Mash and mix the ingredients together. Set aside.
Preheat the oven to 350 degrees F.
Lightly dust the pie crust with some cinnamon and nutmeg. Bake until light brown, about 10 minutes.
Remove the pan from the oven and add all of the sweet potato filling. Roll out one of the pie dough pieces (reserve the other for another use) and slice into strips. Line the top of the pie with parallel strips of pie dough. Rotate the pan 90 degrees and arrange a second row of pie dough at a right angle to the first to create a checked look on top. Brush the top with the egg yolk and bake until the pie is firm, about 40 minutes.